1. Field of the Invention
This invention relates generally to light emitting devices, and more particularly to a light emitting diode (LED) or laser diode coated with phosphors.
2. Description of the Related Art
Solid state light sources have been available for many years. The colors have been predominantly red, orange, and green. Within the last several years blue LEDs have also been produced. The combination of red, green and blue LEDs has enabled the generation of white light with a complicated 4 lead package. However, the different LEDs have different lumen vs life curves causing the color to change with life. In the Fall of 1996, Nichia of Japan introduced a new white LED, Product No. NSPW310AS. This product uses yttrium aluminum garnet doped with cerium to convert blue emission from the LED into a very broad band yellow emission. The emission peak is at 580 nm with a full width at half maximum of 160 nm. The phosphor is coated directly on the LED. The entire device is encapsulated in a clear plastic lens. The emission contains enough orange emission to produce white light at a color temperature of about 8000xc2x0 K., a color rendering index (CRI) of about 77, and a device luminous efficacy of approximately 5 lumens per watt (lm/w) of input electric power.
It is well know that tastes in lighting characteristics can vary by geographical region. For example, in some regions of the world, high color temperatures are preferred, while in other regions lower color temperatures are desired. Similarly, in some regions, CRIs in the mid-nineties are preferred, while in other regions CRIs in the mid-eighties are preferred. Hence, it is desirable to adapt the output characteristics of a lamp to suit particular preferences.
The color temperature of the white LED produced by Nichia can be lowered by using more phosphor, however, the system efficiency is also decreased. There is a need for a phosphor-coated LED having a higher device luminous efficacy; there is also a need for such an LED having a lower color temperature.
A light-emitting device, according to an exemplary embodiment of the invention, comprises a blue-emitting LED covered with a phosphor-containing covering, the phosphor-containing covering containing green-emitting phosphors and red-emitting phosphors, said green and red phosphors being excitable by said blue-emitting LED.
The invention also relates to a phosphor composition which absorbs radiation having a first spectrum and emits radiation having a second spectrum and which comprises at least one of: YBO3:Ce3+,Tb3+; BaMgAl10O17:Eu2+, Mn2+; (Sr,Ca,Ba)(Al,Ga)2S4:Eu2+; and Y3Al5O12xe2x80x94Ce3+; and at least one of: Y2O2S:Eu3+,Bi3+; YVO4:Eu3+,Bi3+; SrS:Eu2+; SrY2S4:Eu2+; (Ca,Sr)S:Eu2+; and CaLa2S4:Ce3+ (where the element following the colon represents an activator).
The invention also relates to a novel phosphor composition comprising Y2O2S:Eu3+,Bi3+.
The invention also relates to a novel phosphor composition comprising YVO4:Eu3+,Bi3+.
The invention also relates to a lamp comprising a phosphor composition and a light emitting device such as an LED or laser diode. The LED or laser diode emits radiation having a first spectrum, at least a portion of which is absorbed by the phosphor composition. The phosphor composition emits radiation having a second spectrum and comprises at least one of: YBO3:Ce3+,Tb3+; BaMgAl10O17:Eu2+,Mn2+; (Sr,Ca,Ba)(Al,Ga)2S4:Eu2+; and Y3Al5O12xe2x80x94Ce3+; and at least one of: Y2O2S:Eu3+,Bi3+; YVO4:Eu3+,Bi3+; SrS:Eu2+; SrY2S4:Eu2+; (Ca,Sr)S:Eu2+; and CaLa2S4:Ce3+.
The invention also relates to a light emitting device comprising a blue emitting LED and a phosphor composition comprising SrS:Eu2+,Ce3+,K+. The SrS:Eu2+,Ce3+,K+ phosphor, when excited by blue light, emits a broadband spectrum including red light and green light.
The invention also relates to a method of producing white light comprising the steps of generating light, directing the light to a phosphor composition comprising at least one of: YBO3:Ce3+,Tb3+; BaMgAl10O17:Eu2+,Mn2+; (Sr,Ca,Ba)(Al,Ga)2S4:Eu2+; and Y3Al5O12xe2x80x94Ce3+; and at least one of: Y2O2S:Eu3+,Bi3+; YVO4:Eu3+,Bi3+; SrS:Eu2+; SrY2S4:Eu2+; (Ca,Sr)S:Eu2+; and CaLa2S4:Ce3+; and converting at least a portion of the light with the phosphor composition to light having a different spectrum.
The phosphor composition and the light source together can produce white light with pleasing characteristics, such as a color temperature of 3000-4100xc2x0 K., a color rendering index of greater than 70, typically greater than 80, for example about 83-87, and a device luminous efficacy of about 10-20 lumens per watt of input electric power.